1. Field of the Invention
The present invention relates to a surface acoustic wave filter device that is employed for example in a mobile communication instrument and so on such as a cellular-phone. Further, the present invention relates to, in particular, a resonant surface acoustic wave filter device that is provided with an inter-digital transducer (IDT) and reflectors. Still further, the present invention relates to a structure of the IDT in a resonant surface acoustic wave filter device that comprises the IDT and the reflectors.
2. Description of the Related Art
Recently, surface acoustic wave filter devices, being small in size, light in weight, highly reliable and excellent in signal attenuation characteristics outside a pass band, are used in large part in the field of mobile communication. In particular, resonant filter devices that utilize the resonance characteristics of surface acoustic wave can be made small in size, and are capable of producing the resonant filter devices for VHF band and UHF band and further excellent in their filter characteristics. Therefore, they are in the mainstream of the surface acoustic wave filter device technology for mobile communication.
The resonant surface acoustic wave filter device employs a surface acoustic wave resonant element such as illustrated schematically in FIG. 40. The surface acoustic wave resonant filter device comprises IDTs and grating reflectors (GRs) formed on a piezoelectric substrate, and in the device, two grating reflectors disposed an appropriate distance apart form a standing wave resonator of the surface acoustic wave. And IDTs are disposed between the two grating reflectors so as to couple well with the standing wave.
Accompanying further progress of communication instruments for such as vehicular communication, still higher characteristics are demanded for the surface acoustic wave filter devices that are used for such communication instruments. Lower loss in a pass band frequency range and in broader frequency ranges, larger attenuation outside a pass band and excellent cut-off characteristics outside the pass band are desired as frequency characteristics of the surface acoustic wave filter devices.
In resonant surface acoustic wave filter devices, surface acoustic wave reflections by IDT electrode fingers are positively utilized in addition to the reflections due to the GRs. An existing IDT comprises solid electrodes in which polarity of the electrode fingers alternates every adjacent electrode finger. Accordingly, characteristics of the resonant surface acoustic wave filter device are determined by a number of the IDT fingers and GRs, a pitch and a thickness thereof and so on.
However, in an existing method in which the characteristics of the filter are obtained by selecting a number, a pitch and a thickness of IDT fingers and GR, there are restrictions due to a chip size and a limitation in the photo-etching process. Further, in the existing method, the impedance of the filter device varies with the change of these parameters, although it is preferable to have the impedance matching between the filter device and an external circuit connected to the filter device. Thus, there is a problem that a resonant surface acoustic wave filter device having desired characteristics is difficult to be obtained.
In addition, an existing surface acoustic wave filter device having steep cut off characteristics can be constituted in multi-stage. In this case, the filter devices of multi-stage are tuned by use of an external circuit element such as a coil or a capacitor, or tuning elements disposed between filter elements so as to make susceptance components of the filter devices of multi-stage small and favorably zero or flat with respect to frequency dependence to obtain preferable filter device characteristics. There are further problems such that the addition of these external tuning elements makes the external circuit complicated when the filter device is employed and makes the filter device itself large.
Furthermore, a surface acoustic wave filter device of the type to which the present invention relates shows improved filter cut off characteristics in which an IDT of the filter device comprises electrode fingers having a different finger pitch or width from the pitch or width for other ordinary electrode fingers. However, in order to provide surface acoustic wave filters which satisfy recent demands, further improvements on filter cut off characteristics are desired.